Very bad - purposely or accidentally - it causes birth defects. Have you ever seen "Deliverance"?

As a dog breeder I can tell you that it is not nearly as taboo in breeding dogs as it is in human reproduction. The main danger of inbreeding is that it doubles genes so it increases the probability of genetic disorders. But it doesnt automatically insure them as some people think. Here is a good reference paragraph on that from a breeder with decades of experience: "It is true inbreeding can create horrific genetic mistakes or it can be the best and fastest way to improve and standardize a line by doubling up on them and making them express themselves. Note this is a tool used to reveal hidden defects and these defectives should be culled from the breeding pool. In the second sense, when your line is heterotrophic for beneficial traits, you can do inbreeding to make them homozygous (set the trait). Note it must be a tremendously important trait. So here inbreedings are done to verify the superiority of a particular strain or to check for recessive problems. Any weakness, faults, deformities, etc. are likely to show up. If a truly superior line or strain has been developed, containing only desirable qualities, these qualities will be seen in the resulting offspring." -http://www.riospitbull.com/methods_of_breeding.htm

Taboo or not, it's still commonly done to bring out traits.

I think that it's terrible to put ur dogs inbreed! That is, of course, unless they're frankfurters. And then to do it perposely? Oy gevalt! Well I think that most photographers would think that forcing ur dogs inbreed perposely is both cruel and unnecessary. I would think that a per-session-ly basis is much more efficient and you'll get a better pose! However I'm still can't figure out why one would want to put ur dogs inbreed at all - perposely or not! Maybe a breeder can explain to me if there's ever a purpose in doing so with your dogs.